Process for producing non quenched and tempered steel

ABSTRACT

A process for producing a non quenched and tempered steel. The process comprises a cooling step at least after a finish rolling step. In said cooling step, an intense cooling and a moderate cooling are carried out alternately to allow the steel to undergo at least two stages of water cooling, so that the core temperature and the surface temperature of the steel become the same within a specified time, and thus ensures the uniformity of the mechanical properties of the steel and improves the production efficiency.

TECHNICAL FIELD

The present invention relates to a process for producing non quenchedand tempered steel, and belongs to the technical field of ferrousmetallurgy.

BACKGROUND

Currently, bars for machine cutting in China generally use ordinarysteels, such as 45, 40Cr and 42CrMo steels. These bars need quenchingand tempering heat treatment when used as raw materials for machinecutting. Quenching and tempering processes are high in cost. Sincequenching and tempering processes increase energy consumption, pollutethe environment, and bring some waste product loss, they do not meet theenergy-saving and environmental protection requirements at present.Therefore, non quenched and tempered steels that do not requirequenching and tempering processes and can be directly used for cuttingwill gradually replace ordinary steels and become a trend in the future.Non quenched and tempered steels means mechanical structural steels thatcan achieve performance requirements without quenching and temperingtreatment. Using such steels to manufacture parts can omit quenching andtempering heat treatment process, and has advantages such as savingenergy and materials, simple technology, etc., and can decreaseenvironmental pollution and prevent oxidation, decarbonization,deformation and cracking.

The domestic traditional process for producing non quenched and temperedsteels for cutting includes: electric furnace smelting—refining —moldcasting—controlled rolling and controlled cooling. The difficulty ofsuch process in production is the control of macrostructure of castingblanks. At present, the domestic and oversea manufacturers generallyachieve the control of macrostructure of casting blanks by improvingchemical composition of non quenched and tempered steels. However, asshown by studies, it is difficult to make non quenched and temperedsteels achieve performance requirements only by component design.

Therefore, the Shougang Corporation has proposed a novel process forproducing non quenched and tempered steels, which process comprises:converter smelting, slag cutoff tapping, deoxidation alloying in ladle,LF ladle refining, feeding S line, full protection casting by ladlebottom blowing argon, slab temperature control, controlled cooling androlling, etc. In the rolling step, the heating temperature is 1100-1180°C., initial rolling temperature is 1020-1100° C., finishing rollingtemperature is 850-920° C., the relative deformation is 15-35%. Afterrolling, the temperature is decreased to 600° C. and then slowly cooledto room temperature. The non quenched and tempered steels produced byabove process are difficult to ensure the core temperature and thesurface temperature of the steels to become the same by slow coolingwithin a short time, and tend to cause the surface and core of thesteels have large differences in strength and toughness, and seriouslynonuniform mechanical properties within a short time. When the aboveprocess is used to produce large-sized non quenched and tempered steels(e.g. φ70 to φ145 mm bars), the phenomenon of nonuniform mechanicalproperties of the surface and the core of the bars is even more obvious.

SUMMARY OF THE INVENTION

The technical problem to be solved by the present invention is toovercome the defects that the surface mechanical properties and coremechanical properties of the steels produced by existing processes forproducing non quenched and tempered steels are nonuniform. Therefore,the present invention provides a process for producing non quenched andtempered steels to ensure the uniformity of the surface mechanicalproperties and core mechanical properties of final products.

For this purpose, the present invention provides a process for producingnon quenched and tempered steel, comprising a cooling step at leastcarried out after a finishing step; in said cooling step, an intensecooling and a moderate cooling are carried out alternately to allow thesteel to undergo at least two stages of water cooling, so that the coretemperature and the surface temperature of the steel become the samewithin a specified time.

According to the process for producing non quenched and tempered steelof present invention, in said cooling step, the steel is subjected tothree stages of water cooling, wherein the first stage of water coolinguses intense cooling, the second stage of water cooling uses moderatecooling, and the third stage of water cooling uses intense cooling.

According to the process for producing non quenched and tempered steelof present invention, in said cooling step, the cooling intensity iscontrolled by controlling the opening degree of valve(s) of a watercooling unit.

According to the process for producing non quenched and tempered steelof present invention, in said cooling step, the temperature of the steeldecreases 100° C.-400° C. in 4-7 seconds after water cooling, and thetemperature of the steel further decreases 50° C.-200° C. aftertemperature reversion.

According to the process for producing non quenched and tempered steelof present invention, in said cooling step, the first stage valveopening degree is controlled to be 30%-40%, the second stage valveopening degree is controlled to be 20%, and the third stage valveopening degree is controlled to be 30%-40%, to ensure the surfacetemperature of the steel to decrease 100-400° C. in 4-7 seconds.

According to the process for producing non quenched and tempered steelof present invention, in said cooling step, the steel is subjected totemperature-dropping cooling by means of spray cooling after temperaturereversion.

According to the process for producing non quenched and tempered steelof present invention, after said temperature-dropping cooling, the steelis separately disposed on a cold bed and subjected to air cooling for10-12 minutes.

According to the process for producing non quenched and tempered steelof present invention, after said air cooling, the steel is stacked andsubjected to shield cooling.

The process for producing non quenched and tempered steel according topresent invention further comprises a finish rolling step before thecooling step, in said finish rolling step, the temperature of the steelis controlled at 950° C. at the entry into the finish rolling step; thesteel is subjected to low temperature rolling when the steel temperatureis 780° C.-900° C.

The process for producing non quenched and tempered steel according topresent invention further comprises a smelting step before the finishrolling step, the smelting step comprises electric furnace smeltingstep, ladle furnace smelting step and refining step carried outsequentially.

According to the process for producing non quenched and tempered steelof present invention, a molten iron smelting is utilized in the electricfurnace smelting, wherein the final phosphorus content is ≦0.015%, thefinal carbon content is 0.03% to 0.10%, and the final temperature is1620-1700° C.

According to the process for producing non quenched and tempered steelof present invention, in the ladle furnace smelting step and/or therefining step silicon carbide, ferrosilicon powder are used todeoxidize.

According to the process for producing non quenched and tempered steelof present invention, the ladle furnace smelting step comprises makingwhite slag, and holding the white slag for 20 minutes or more.

According to the process for producing non quenched and tempered steelof present invention, in the refining step, the refining time is 45minutes or more, and the hydrogen content is controlled at 1.5 ppm orless.

The process for producing non quenched and tempered steel of presentinvention further comprises a continuous casting step after the refiningstep, in said continuous casting step, a overheat is controlled at20-35° C., a pulling speed is controlled at 0.5 m/min-0.6 m/min.

The process for producing non quenched and tempered steel of presentinvention further comprises a heating step after the continuous castingstep, in said heating step, the steel billet is placed in a heatingfurnace to be heated, wherein the preheating stage temperature iscontrolled at 850±30° C., the heating stage temperature is controlled at1100±30° C., the soaking stage temperature is controlled at 1130±30° C.,and the total time of the soaking stage is not less than 2 hours.

In a preferred embodiment, the present invention provides a process forproducing a non quenched and tempered steel, the process comprisesfollowing steps, successively:

(1) an electric furnace smelting step: providing iron raw materialshaving desired steel composition, a molten iron smelting is utilized inthe electric furnace smelting , wherein final phosphorus content is≦0.015%, final carbon content is 0.03% to 0.10%, and final temperatureis 1620° C.-1700° C.;

(2) a ladle furnace smelting step: wherein deoxidation is carried outusing silicon carbide and/or ferrosilicon powder, white slag is made byadding lime, the white slag is held for 20 minutes or more;

(3) a refining step: wherein deaeration is carried out, the refiningtime is controlled at 45 minutes or more, and the hydrogen content iscontrolled at 1.5 ppm or less;

(4) a continuous casting step: the steel melt obtained in the refiningstep is subjected to continuous casting, the overheat of the steel meltis controlled at 20-35° C., the pulling speed is controlled at 0.5m/min-0.6 m/min;

(5) a heating step: the steel billet produced in the continuous castingstep is placed in a furnace to be heated, wherein the preheating stagetemperature is controlled at 850±30° C., the heating stage temperatureis controlled at 1100±30° C., the soaking stage temperature iscontrolled at 1130±30° C., the total time of the soaking stage is notless than 2 hours;

(6) a finish rolling step: wherein the temperature of the steel iscontrolled at 950° C. at the entry into the finish rolling step; thesteel is subjected to low temperature rolling when the steel temperatureis 780-900° C.; and

(7) a cooling step: wherein intense cooling and moderate cooling arecarried out alternately to allow the steel to undergo at least twostages of water cooling, so that the core temperature and the surfacetemperature become the same during the cooling.

The process for producing non quenched and tempered steel according topresent invention has following advantages:

1. The process for the production of non quenched and tempered steelaccording to present invention alters the cooling mode before finishrolling in previous production of non quenched and tempered steel; theprocess at least has a cooling step after the finish rolling step; incontrast to the cooling mode in the prior art which employs the coolingmode of single water cooling or consistent air cooling, the presentprocess utilizes alternate intense cooling and moderate cooling. Theintense cooling can ensure the surface temperature of the steel todecrease rapidly; and the moderate cooling allows the core temperatureof the steel to dissipate gradually to the surface; a further intensecooling is carried out to allow rapid heat dissipation. The intensecooling and the moderate cooling can be carried out alternately severaltimes according to practical requirement. A water cooling mode combiningintense cooling and moderate cooling allows the core temperature and thesurface temperature of the steel to become the same within a short time,and thus ensures the uniformity of the mechanical properties of thesteel and improves the production efficiency.

2. According to the process for producing non quenched and temperedsteel of present invention, in said cooling step, the steel is subjectedto three stages of water cooling, wherein the first stage of watercooling uses intense cooling, the second stage of water cooling usesmoderate cooling, and the third stage of water cooling uses intensecooling. Upon the completion of the finish rolling, the steel has arelatively high temperature. An intense cooling is utilized in the firststage of water cooling so that the surface temperature of the steeldecreases rapidly. Due to the heat transfer effect, after the surfacetemperature decreases, the heat of the core gradually transfers to thesurface. In order to make the heat of the core transfers to the surfaceas much as possible, a moderate cooling is utilized in the second stageof water cooling. After the moderate cooling, heat transfer allows thesurface temperature to increase, and then the surface is cooled rapidlyagain by intense cooling mode so that the surface heat is removedquickly. At this time, the heat transfer allows the surface temperatureand the core temperature become the same, thus ensuring the uniformityof mechanical properties.

3. According to the process for producing non quenched and temperedsteel of present invention, in said cooling step, the cooling iscontrolled by controlling the opening degree of valves of a watercooling unit, in particular, the first stage valve opening degree iscontrolled to be 30%-40%, the second stage valve opening degree iscontrolled to be 20%, and the third stage valve opening degree iscontrolled to be 30%-40% to ensure the surface temperature of the steelto decrease 100° C.-400° C. in 4-7 seconds. The flow rate of water canbe controlled by controlling the opening degree of valve, such that theintensity of water cooling can be controlled. This control manner isvery simple. After the valve opens a certain length, the steel isintroduced into water to perform water treatment. During the watertreatment, the surface of the steel is cooled in all directions toensure the uniformity of surface cooling.

4. According to the process for producing non quenched and temperedsteel of present invention, in said cooling step, the steel is subjectedto temperature-dropping cooling by means of spray cooling aftertemperature reversion. Spray cooling is an advantageous supplement ofwater cooling. By spray cooling, the heat of the core further spreads tothe surface, which further ensures the core temperature and the surfacetemperature become the same.

5. According to the process for producing non quenched and temperedsteel of present invention, after said temperature-dropping cooling, thesteel is separately disposed on a cold bed and subjected to air coolingfor 10-12 minutes. After spray cooling, the steel is separately disposedon a cold bed to perform air cooling. A spray cooling can beadditionally provided to further dissipate surface heat.

6. According to the process for producing non quenched and temperedsteel of present invention, after said air cooling, the steel is stackedto perform shield cooling. Shield cooling is a manner of slow cooling.In order to avoid the above cooling procedure too rapid and impairingthe structure and property of the steel, the steel is stacked andsubjected to shield cooling. After the cooling scheme including watercooling, spray cooling and air cooling, the surface temperature and thecore temperature of the steel has essentially become the same. At thistime, shield cooling is used to slow down the cooling speed, therebyimproving the structure and property of the steel.

7. According to the process for producing non quenched and temperedsteel of present invention, in said ladle furnace smelting step, whiteslag is made and held for 20 minutes or more. The white slag holdingtime is controlled strictly to make the effect of deoxidation,desulfurization and inclusion removal of the white slag more obvious,thereby improving the purity of steel.

8. According to the process for producing non quenched and temperedsteel of present invention, in the refining step, the refining time isnot less than 45 minutes, and the hydrogen content is controlled at 1.5ppm or less. The refining process effectively controls the hydrogencontent and can better solve the risk of subsequent hydrogen inducedcracking of steel; there is an adequate time to make a more uniformcomposition; and the process provides an adequate time for inclusions tofloat upward, thereby effectively solving the problem of controlling theinclusions, such that the finished product will be purer.

9. According to the process for producing non quenched and temperedsteel of present invention, in the continuous casting step, the overheatis strictly controlled at 20-35° C.; the pulling speed is controlled at0.5 m/min-0.6 m/min. In the continuous casting, a low overheat and a lowpulling speed ensure the quality of casting blanks.

The keypoint of present invention is to make the performance of thesurface and core of steels substantially consistent by controlling therolling and by controlling the cooling step after the rolling, therebyimproving the quality of steels. Specific cooling control includes:

-   -   (1) After finish rolling, the steel is subjected to at least two        stages of water cooling by alternating intense cooling and        moderate cooling, such that the core temperature and the surface        temperature of the steel become the same within a certain time.        In particular, after finish rolling, said steel is subjected to        three stages of water cooling, wherein the first stage of water        cooling uses intense cooling, the second stage of water cooling        uses moderate cooling, and the third stage of water cooling uses        intense cooling. In the specific water cooling, the intensity of        cooling is controlled by controlling the opening degree of        valve(s) of the water cooling unit. In present invention, the        intense cooling generally means a cooling at a cooling rate of        ≧7° C./s; whereas the moderate cooling means a cooling at a        cooling rate of 2-4° C./s.    -   (2) After water cooling, the steel is annealed and then        subjected to temperature-dropping cooling by means of spray        cooling after temperature reversion.    -   (3) After said temperature-dropping cooling, the steel is        separately disposed on a cold bed and subjected to air cooling        for 10-12 minutes.    -   (4) After said air cooling, the steel is stacked and subjected        to shield cooling.

After finish rolling, the steel is subjected to cooling control throughabove manner (especially water cooling), which alters the cooling modebefore finish rolling in previous production of non quenched andtempered steel; the process at least has a cooling step after the finishrolling step; in contrast to the cooling mode in the prior art whichemploys the cooling mode of single water cooling or consistent aircooling, the present process utilizes alternate intense cooling andmoderate cooling. The intense cooling can ensure the surface temperatureof the steel to decrease rapidly; and the moderate cooling allows thecore temperature of the steel to dissipate gradually to the surface; afurther intense cooling is carried out to allow rapid heat dissipation.The intense cooling and the moderate cooling can be carried outalternately several times according to practical requirement. A watercooling mode combining intense cooling and moderate cooling allows thecore temperature and the surface temperature of the steel to become thesame within a short time, and thus ensures the uniformity of themechanical properties of the steel and improves the productionefficiency. On such a basis, by a combination control of subsequentspray cooling, air cooling and shield cooling allows the coretemperature to continuously dissipate to the surface and the surfacetemperature to be taken away constantly; moreover, the combination ofsaid cooling manners results in an appropriate cooling speed. Use ofshield cooling after air cooling allows the cooling speed not so fast inthe case that the surface temperature and the core temperature of steelare consistent, thus improving overall mechanical properties.

DESCRIPTION OF THE DRAWINGS

In order to make present invention to be clearly understood more easily,the present invention will be further described in detail according tothe embodiments with reference to the drawings, wherein:

FIG. 1 is a metallograph at a magnification of 500X of a non quenchedand tempered steel produced by the manufacturing process of presentinvention for replacing common quenched and tempered 45 steel;

FIG. 2 is an image showing the grain size of a non quenched and temperedsteel produced by the manufacturing process of present invention forreplacing common quenched and tempered 45 steel;

FIG. 3 is an image showing inclusions of a non quenched and temperedsteel produced by the manufacturing process of present invention forreplacing common quenched and tempered 45 steel;

FIG. 4 is a metallograph at a magnification of 500X of a non quenchedand tempered steel produced by the manufacturing process of presentinvention for replacing quenched and tempered 42CrMo steel;

FIG. 5 is an image showing the grain size of a non quenched and temperedsteel produced by the manufacturing process of present invention forreplacing quenched and tempered 42CrMo steel;

FIG. 6 is an image showing inclusions of a non quenched and temperedsteel produced by the manufacturing process of present invention forreplacing quenched and tempered 42CrMo steel.

DETAILED DESCRIPTION

The following description will explain the controlled rolling,controlled cooling, smelting and continuous casting steps of presentinvention in detail, with respect to the production processes of severalnon quenched and tempered steels.

Example 1

This example provides a process for producing a non quenched andtempered steel which is in place of common quenched and tempered 45steel and directly for cutting application. The process comprises afinish rolling step and a cooling step after the finish rolling;wherein, in the finish rolling step, the temperature of rods at theentry into the finish rolling step was controlled at 950° C.; the rodswere subjected to low temperature rolling when the rods temperature is780° C.-900° C. After the rolling, the steel was subjected to threestages of water cooling by means of a specialized controllable watercooling unit, wherein the first stage of water cooling employed intensecooling, the second stage of water cooling employed moderate cooling,and the third stage of water cooling employed intense cooling.

Here, it should be noted that there are many ways to control watercooling intensity. In this example, the water flow was controlled bycontrolling the opening degree of the valve(s) of water cooling unit soas to control water cooling strength. Specifically, the first stagevalve opening degree was controlled to be 30%-40%, the second stagevalve opening degree was controlled to be 20%, and the third stage valveopening degree was controlled to be 30%-40% to ensure the surfacetemperature of the rods to decrease 100° C.-400° C. in 5 seconds. Afterthe temperature of the rods was reversed, the rods were subjected tospray cooling to decrease the rods temperature by 50-200° C., such thatthe heat quickly dissipates, then the rods were separately disposed on acold bed and subjected to air cooling for 10 minutes, finally the rodswere removed from the cold bed and stacked to undergo shield cooling.

In the process of this example for producing a non quenched and temperedsteel which is in place of common quenched and tempered 45 steel anddirectly for cutting application, the rods were subjected to threestages of water cooling, wherein the first stage of water coolingemployed intense cooling, the second stage of water cooling employedmoderate cooling, and the third stage of water cooling employed intensecooling. Upon the completion of finish rolling, the rods had arelatively high temperature. The rods were subjected to the first stageof water cooling using intense cooling so that the surface temperatureof the steel decreased rapidly. After the surface temperature decreased,the heat of the core gradually transferred to the surface, due to theheat transfer effect. In order to make the heat of the core transfer tothe surface as much as possible, the second stage of water coolingemployed moderate cooling so that more time was left for heat transferof the core during cooling. After the moderate cooling, the heattransfer allowed the surface temperature increasing, and then thesurface was cooled rapidly again by intense cooling manner so that theheat of the surface was removed quickly. At this time, the heat transferallowed the surface temperature and the core temperature become thesame, thus ensuring the uniformity of mechanical properties.

Example 2

This example provides a process for producing a non quenched andtempered steel which is in place of common quenched and tempered 45steel and directly for cutting application; the process is a furtherimprovement over Example 1. As compared with Example 1, this processfurther comprises a smelting step before the finish rolling step, thesmelting step comprising electric furnace smelting step, ladle furnacesmelting and refining step carried out sequentially.

In the electric furnace smelting step, molten iron smelting wasemployed, the phosphorus content before steel tapping was strictlycontrolled at ≦0.015%, the final carbon content was 0.03% to 0.10%, andthe final temperature was 1620° C.-1700° C. The electric furnacesmelting can better control deslagging operation than traditionalconverter smelting.

In the ladle furnace (LF) smelting step, deoxidization was performedusing silicon carbide, ferrosilicon powder; lime was added to make whiteslag; the white slag was held for 20 minutes or more so that theinclusions can be thoroughly removed by the white slag.

In the refining furnace (VD) smelting step, degassing treatment wascarried out to ensure that hydrogen content was controlled at 1.5 ppm orless, and the refining time was not less than 45 minutes.

Advantages of using LF furnace+VD furnace refining: as compared withtraditional process using LF furnace refining only, this refiningprocess effectively controls hydrogen content, and can better solve therisk of subsequent hydrogen induced cracking of bars; an adequate timeallows to obtain a more uniform composition; and an adequate time isprovided for inclusions to float upward, thus effectively solving theproblem of inclusion control.

Example 3

This example provides a process for producing a non quenched andtempered steel which is in place of common quenched and tempered 45steel and directly for cutting application; the process is a furtherimprovement over Example 1 and Example 2. In this example, thecontinuous casting step and heating step were improved. The continuouscasting step and the heating step were arranged after the refining step,and before the rolling step and the water cooling step.

In the continuous casting step, molten iron in a tundish was introducedinto a crystallizer by a submerged nozzle, thus the problem that airtends to be brought in when using traditional nozzles is avoided.Furthermore, argon gas was blow to the joint site of the submergednozzle and the tundish, thus preventing air from entering into thetundish. The overheat was strictly controlled at 20-35° C., and thepulling speed was controlled at 0.5 m/min-0.6 m/min. In the continuouscasting, low overheat and low casting speed ensured the quality ofcasting blank. When cutting after continuous casting, the temperature atcut place was controlled at 820° C. After cutting, the surface of thecasting blank needed to be manually checked to ensure no obviousdefects. A macrostructure sample of the casting blank was taken toensure that the casting blank had no cracks or shrinkage. The centrallooseness was not more than level three; this requirement was to ensurethe quality of the surface and macrostructure of the rods that aresubsequently rolled. After passing inspection, the casting blank wassent to a heating furnace to be heated, wherein the preheating stagetemperature was 850±30° C., the heating stage temperature was 1100±30°C., the soaking stage temperature was 1130±30° C., and the total time ofthe soaking stage was not less than 2 hours.

The process in this example produced a non quenched and tempered steelwhich is in place of common quenched and tempered 45 steel and directlyfor cutting application, its metallographic structures at amagnification of 500X comprise ferrite and pearlite (as shown in FIG.1), and have actual grain size (100X) rated to level 10 to 11 accordingto GB/T6394 (as shown in FIG. 2); the grains are fine and uniform; andthe rate difference from the core to the edge is not greater than 1.5.The mechanical properties of the surface and the core of steel areuniform. The strength and the toughness from the core to the edge changevery little, thus effectively avoiding the defects that the mechanicalproperties of general materials cannot satisfy application demands aftersubjecting to large surface processing. The hardness difference from thecore to the edge is less than 30 HB, thus effectively avoiding thedisadvantageous effect of large hardness changes on cutting tools andprocessing. Moreover, the content of inclusions is low, and the purityof the steel is high (as shown in FIG. 3).

Example 4

This example provides a process for producing a non quenched andtempered steel which is in place of quenched and tempered 42CrMo steeland directly for cutting application. The process comprises a finishrolling step and a cooling step after the finish rolling; wherein, inthe finish rolling step, the temperature of rods at the entry into thefinish rolling step was controlled at ≦900° C.; the rods were subjectedto low temperature rolling when the rods temperature is 800° C.-900° C.After the rolling, the steel was subjected to three stages of watercooling by means of a specialized controllable water cooling unit,wherein the first stage of water cooling employed intense cooling, thesecond stage of water cooling employed moderate cooling, and the thirdstage of water cooling employed intense cooling.

Here, it should be noted that there are many ways to control watercooling intensity. In this example, the water flow was controlled bycontrolling the opening degree of the valve(s) of water cooling unit soas to control water cooling strength. Specifically, the first stagevalve opening degree was controlled to be 30%-40%, the second stagevalve opening degree was controlled to be 30%, and the third stage valveopening degree was controlled to be 30%-40% to ensure the surfacetemperature of the rods to decrease 150° C.-400° C. in 5 seconds. Afterthe temperature of the rods was reversed, the rods were subjected tospray cooling to decrease the rods temperature by 80-200° C., such thatthe heat quickly dissipates, then the rods were separately disposed on acold bed and subjected to air cooling for 10 minutes, finally the rodswere removed from the cold bed and stacked to undergo shield cooling.

In the process of this example for producing a non quenched and temperedsteel which is in place of quenched and tempered 42CrMo steel anddirectly for cutting application, the rods were subjected to threestages of water cooling, wherein the first stage of water coolingemployed intense cooling, the second stage of water cooling employedmoderate cooling, and the third stage of water cooling employed intensecooling. Upon the completion of finish rolling, the rods had arelatively high temperature. The rods were subjected to the first stageof water cooling using intense cooling so that the surface temperatureof the steel decreased rapidly. After the surface temperature decreased,the heat of the core gradually transferred to the surface, due to theheat transfer effect. In order to make the heat of the core transfer tothe surface as much as possible, the second stage of water coolingemployed moderate cooling so that more time was left for heat transferof the core during cooling. After the moderate cooling, the heattransfer allowed the surface temperature increasing, and then thesurface was cooled rapidly again by intense cooling manner so that theheat of the surface was removed quickly. At this time, the heat transferallowed the surface temperature and the core temperature become thesame, thus ensuring the uniformity of mechanical properties.

Example 5

This example provides a process for producing a non quenched andtempered steel which is in place of quenched and tempered 42CrMo steeland directly for cutting application; the process is a furtherimprovement over Example 1. As compared with Example 1, this processfurther comprises a smelting step before the finish rolling step, thesmelting step comprising electric furnace smelting step, ladle furnacesmelting and refining step carried out sequentially.

In the electric furnace smelting step, molten iron smelting wasemployed, the phosphorus content before steel tapping was strictlycontrolled at ≦0.015%, the final carbon content was 0.03% to 0.10%, andthe final temperature was 1670° C.-1700° C. The electric furnacesmelting can better control deslagging operation than traditionalconverter smelting.

In the ladle furnace (LF) smelting step, deoxidization was performedusing silicon carbide, ferrosilicon powder; lime was added to make whiteslag; the white slag was held for 20 minutes or more so that theinclusions can be thoroughly removed by the white slag.

In the refining furnace (VD) smelting step, degassing treatment wascarried out to ensure that hydrogen content was controlled at 1.5 ppm orless, and the refining time was not less than 45 minutes.

Advantages of using LF furnace+VD furnace refining: as compared withtraditional process using LF furnace refining only, this refiningprocess effectively controls hydrogen content, and can better solve therisk of subsequent hydrogen induced cracking of bars; an adequate timeallows to obtain a more uniform composition; and an adequate time isprovided for inclusions to float upward, thus effectively solving theproblem of inclusion control.

Example 6

This example provides a process for producing a non quenched andtempered steel which is in place of quenched and tempered 42CrMo steeland directly for cutting application; the process is a furtherimprovement over Example 1 and Example 2. In this example, thecontinuous casting step and heating step were improved. The continuouscasting step and the heating step were arranged after the refining step,and before the rolling step and the water cooling step.

In the continuous casting step, molten iron in a tundish was introducedinto a crystallizer by a submerged nozzle, thus the problem that airtends to be brought in when using traditional nozzles is avoided.Furthermore, argon gas was blow to the joint site of the submergednozzle and the tundish, thus preventing air from entering into thetundish. The overheat was strictly controlled at 23-35° C., and thepulling speed was controlled at 0.5 m/min-0.6 m/min. In the continuouscasting, low overheat and low casting speed ensured the quality ofcasting blank. When cutting after continuous casting, the temperature atcut place was controlled at 820° C. After cutting, the surface of thecasting blank needed to be manually checked to ensure no obviousdefects. A macrostructure sample of the casting blank was taken toensure that the casting blank had no cracks or shrinkage. The centrallooseness was not more than level three; this requirement was to ensurethe quality of the surface and macrostructure of the rods that aresubsequently rolled. After passing inspection, the casting blank wassent to a heating furnace to be heated, wherein the preheating stagetemperature was 850±30° C., the heating stage temperature was 1100±30°C., the soaking stage temperature was 1130±30° C., and the total time ofthe soaking stage was not less than 2 hours.

The process in this example produced a non quenched and tempered steelwhich is in place of quenched and tempered 42CrMo steel and directly forcutting application, its metallographic structures at a magnification of500X comprise ferrite and pearlite (as shown in FIG. 3), and have actualgrain size (100X) rated to level 10 according to GB/T6394 (as shown inFIG. 4); the grains are fine and uniform; and the rate difference fromthe core to the edge is not greater than 1.5. The mechanical propertiesof the surface and the core of steel are uniform. The strength and thetoughness from the core to the edge change very little, thus effectivelyavoiding the defects that the mechanical properties of general materialscannot satisfy application demands after subjecting to large surfaceprocessing. The hardness difference from the core to the edge is lessthan 30 HB, thus effectively avoiding the disadvantageous effect oflarge hardness changes on cutting tools and processing. Moreover, thecontent of inclusions is low, and the purity of the steel is high (asshown in FIG. 6).

Example 7

This example provides a general process for producing non quenched andtempered steels, the process begins from smelting step. The smeltingstep comprising electric furnace smelting step, ladle furnace smeltingand refining step carried out sequentially. In the electric furnacesmelting step, molten iron smelting was employed, the phosphorus contentbefore steel tapping was strictly controlled at ≦0.015%, the finalcarbon content was 0.03% to 0.10%, and the final temperature was 1620°C.-1700° C. The electric furnace smelting can better control deslaggingoperation than traditional converter smelting. In the ladle furnace (LF)smelting step, deoxidization was performed using silicon carbide,ferrosilicon powder; lime was added to make white slag; the white slagwas held for 20 minutes or more so that the inclusions can be thoroughlyremoved by the white slag. In the refining furnace (VD) smelting step,degassing treatment was carried out to ensure that hydrogen content wascontrolled at 1.5 ppm or less, and the refining time was not less than45 minutes.

Advantages of using LF furnace+VD furnace refining: as compared withtraditional process using LF furnace refining only, this refiningprocess effectively controls hydrogen content, and can better solve therisk of subsequent hydrogen induced cracking of bars; an adequate timeallows to obtain a more uniform composition; and an adequate time isprovided for inclusions to float upward, thus effectively solving theproblem of inclusion control.

The refining step was followed by a continuous casting step. In thecontinuous casting step, molten iron in a tundish was introduced into acrystallizer by a submerged nozzle, thus the problem that air tends tobe brought in when using traditional nozzles is avoided. Furthermore,argon gas was blow to the joint site of the submerged nozzle and thetundish, thus preventing air from entering into the tundish. Theoverheat was strictly controlled at 20-35° C., and the pulling speed wascontrolled at 0.5 m/min-0.6 m/min. In the continuous casting, lowoverheat and low casting speed ensured the quality of casting blank.When cutting after continuous casting, the temperature at cut place wascontrolled at 820° C. After cutting, the surface of the casting blankneeded to be manually checked to ensure no obvious defects. Amacrostructure sample of the casting blank was taken to ensure that thecasting blank had no cracks or shrinkage. The central looseness was notmore than level three; this requirement was to ensure the quality of thesurface and macrostructure of the rods that are subsequently rolled.After passing inspection, the casting blank was sent to a heatingfurnace to be heated, wherein the preheating stage temperature was850±30° C., the heating stage temperature was 1100±30° C., the soakingstage temperature was 1130±30° C., and the total time of the soakingstage was not less than 2 hours.

The heating step was followed by a finish rolling step and a coolingstep. In the finish rolling step, the temperature of rods at the entryinto the finish rolling step was controlled at 950° C.; the rods weresubjected to low temperature rolling when the rods temperature is 780°C.-900° C. After the rolling, the steel was subjected to three stages ofwater cooling by means of a specialized controllable water cooling unit,wherein the first stage of water cooling employed intense cooling, thesecond stage of water cooling employed moderate cooling, and the thirdstage of water cooling employed intense cooling.

In this example, the water flow was controlled by controlling theopening degree of the valve(s) of water cooling unit so as to controlwater cooling strength. Specifically, the first stage valve openingdegree was controlled to be 30%-40%, the second stage valve openingdegree was controlled to be 20%, and the third stage valve openingdegree was controlled to be 30%-40% to ensure the surface temperature ofthe rods to decrease 100° C.-400° C. in 5 seconds. After the temperatureof the rods was reversed, the rods were subjected to spray cooling todecrease the rods temperature by 50-200° C., such that the heat quicklydissipates, then the rods were separately disposed on a cold bed andsubjected to air cooling for 10 minutes, finally the rods were removedfrom the cold bed and stacked to undergo shield cooling.

In the production process of this example, the rods were subjected tothree stages of water cooling, wherein the first stage of water coolingemployed intense cooling, the second stage of water cooling employedmoderate cooling, and the third stage of water cooling employed intensecooling. Upon the completion of finish rolling, the rods had arelatively high temperature. The rods were subjected to the first stageof water cooling using intense cooling so that the surface temperatureof the steel decreased rapidly. After the surface temperature decreased,the heat of the core gradually transferred to the surface, due to theheat transfer effect. In order to make the heat of the core transfer tothe surface as much as possible, the second stage of water coolingemployed moderate cooling so that more time was left for heat transferof the core during cooling. After the moderate cooling, the heattransfer allowed the surface temperature increasing, and then thesurface was cooled rapidly again by intense cooling manner so that theheat of the surface was removed quickly. At this time, the heat transferallowed the surface temperature and the core temperature become thesame, thus ensuring the uniformity of mechanical properties.

Obviously, the above examples are merely exemplary examples for cleardescription, but not a limitation to the embodiments. Those skilled inthe art can make a change or modification in other forms on the basis ofabove descriptions. It is unnecessary and impossible to list all theembodiments. Obvious changes or modifications derived therefrom arestill within the protection scope of the present invention.

1. A process for producing non quenched and tempered steel, comprising acooling step at least carried out after a finishing step, characterizedin that: in said cooling step, an intense cooling and a moderate coolingare carried out alternately to allow the steel to undergo at least twostages of water cooling, so that the core temperature and the surfacetemperature of the steel become the same within a specified time.
 2. Theprocess for producing non quenched and tempered steel according to claim1, characterized in that: in said cooling step, the steel is subjectedto three stages of water cooling, wherein the first stage of watercooling uses intense cooling, the second stage of water cooling usesmoderate cooling, and the third stage of water cooling uses intensecooling.
 3. The process for producing non quenched and tempered steelaccording to claim 1, characterized in that: in said cooling step, thecooling intensity is controlled by controlling the opening degree ofvalve(s) of a water cooling unit.
 4. The process for producing nonquenched and tempered steel according to claim 1, characterized in that:in said cooling step, the temperature of the steel decreases 100°C.-400° C. in 4-7 seconds after water cooling, and the temperature ofthe steel further decreases 50° C.-200° C. after temperature reversion.5. The process for producing non quenched and tempered steel accordingto claim 4, characterized in that: in said cooling step, the first stagevalve opening degree is controlled to be 30%-40%, the second stage valveopening degree is controlled to be 20%, and the third stage valveopening degree is controlled to be 30%-40%, to ensure the surfacetemperature of the steel to decrease 100-400° C. in 4-7 seconds.
 6. Theprocess for producing non quenched and tempered steel according to claim3, characterized in that: in said cooling step, the steel is subjectedto temperature-dropping cooling by means of spray cooling aftertemperature reversion.
 7. The process for producing non quenched andtempered steel according to claim 3, characterized in that: after saidtemperature-dropping cooling, the steel is separately disposed on a coldbed and subjected to air cooling for 10-12 minutes.
 8. The process forproducing non quenched and tempered steel according to claim 7,characterized in that: after said air cooling, the steel is stacked andsubjected to shield cooling.
 9. The process for producing non quenchedand tempered steel according to claim 1, characterized in that: theprocess further comprises a finish rolling step before the cooling step,in said finish rolling step, the temperature of the steel is controlledat ≦950° C. at the entry into the finish rolling step; the steel issubjected to low temperature rolling when the steel temperature is 780°C.-900° C.
 10. The process for producing non quenched and tempered steelaccording to claim 6, characterized in that: the process furthercomprises a smelting step before the finish rolling step, the smeltingstep comprises electric furnace smelting step, ladle furnace smeltingstep and refining step carried out sequentially.
 11. The process forproducing non quenched and tempered steel according to claim 10,characterized in that: a molten iron smelting is utilized in theelectric furnace smelting, wherein the final phosphorus content is≦0.015%, the final carbon content is 0.03% to 0.10%, and the finaltemperature is 1620-1700° C.
 12. The process for producing non quenchedand tempered steel according to claim 10, characterized in that: in theladle furnace smelting step and/or the refining step silicon carbide,ferrosilicon powder are used to deoxidize.
 13. The process for producingnon quenched and tempered steel according to claim 10, characterized inthat: the ladle furnace smelting step comprises making white slag, andholding the white slag for 20 minutes or more.
 14. The process forproducing non quenched and tempered steel according to claim 13,characterized in that: in the refining step, the refining time is 45minutes or more, and the hydrogen content is controlled at 1.5 ppm orless.
 15. The process for producing non quenched and tempered steelaccording to claim 14, characterized in that: the process furthercomprises a continuous casting step after the refining step, in saidcontinuous casting step, a overheat is controlled at 20-35° C., apulling speed is controlled at 0.5 m/min-0.6 m/min.
 16. The process forproducing non quenched and tempered steel according to claim 15,characterized in that: the process further comprises a heating stepafter the continuous casting step, in said heating step, the steelbillet is placed in a heating furnace to be heated, wherein thepreheating stage temperature is controlled at 850±30° C., the heatingstage temperature is controlled at 1100±30° C., the soaking stagetemperature is controlled at 1130±30° C., and the total time of thesoaking stage is not less than 2 hours.
 17. A process for producing anon quenched and tempered steel, the process comprises following steps,successively: (1) an electric furnace smelting step: providing iron rawmaterials having desired steel composition, a molten iron smelting isutilized in the electric furnace smelting , wherein final phosphoruscontent is ≦0.015%, final carbon content is 0.03% to 0.10%, and finaltemperature is 1620° C.-1700° C.; (2) a ladle furnace smelting step:wherein deoxidation is carried out using silicon carbide and/orferrosilicon powder, white slag is made by adding lime, the white slagis held for 20 minutes or more; (3) a refining step: wherein deaerationis carried out, the refining time is controlled at 45 minutes or more,and the hydrogen content is controlled at 1.5 ppm or less; (4) acontinuous casting step: the steel melt obtained in the refining step issubjected to continuous casting, the overheat of the steel melt iscontrolled at 20-35° C., the pulling speed is controlled at 0.5m/min-0.6 m/min; (5) a heating step: the steel billet produced in thecontinuous casting step is placed in a furnace to be heated, wherein thepreheating stage temperature is controlled at 850±30° C., the heatingstage temperature is controlled at 1100±30° C., the soaking stagetemperature is controlled at 1130±30° C., the total time of the soakingstage is not less than 2 hours; (6) a finish rolling step: wherein thetemperature of the steel is controlled at 950° C. at the entry into thefinish rolling step; the steel is subjected to low temperature rollingwhen the steel temperature is 780-900° C.; and (7) a cooling step:wherein intense cooling and moderate cooling are carried out alternatelyto allow the steel to undergo at least two stages of water cooling, sothat the core temperature and the surface temperature become the sameduring the cooling.